Complexes of N-Macrocycles

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ Ni2+ Fe2+ = Cu2+ Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH 

Cobalt porphyrin derivatives were also reported129 to be active for electrochemical reduction of C02 to formic acid at an amalgamated Pt electrode. More recently, Becker et al have reported130 that Ag2+ and Pd2+ metalloporphyrins acted as homogeneous catalysts for C02 reduction in dry CH2C12 oxalic acid and H2 (its source was not clear) were produced, but no CO was detected. 

Tetraazamacrocyclic complexes131 of cobalt and nickel were found110 to be effective in facilitating the reduction of C02 at -1.3 to -1.6 V versus SCE (Table 8). An acetonitrile-water mixture and water were used as solvents, while in dry dimethylsulfoxide no catalytic reduction of C02 took place. Using an Hg electrode, both CO and H2 were produced, where total current efficiencies were greater than 90%. The turnover numbers of the catalysts were 2-9 h 1. The catalytic activity lasted for more than 24 h and the turnover numbers of the catalysts exceeded 100. A protic source was required to produce both CO and H2, and the authors suggested that both products may arise from a common intermediate, which is most likely a metal hydride. The applied potential for C02 reduction was further reduced by using illuminated p- Si in the presence of the above catalysts.111 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids 

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Aromatic amine complexes65 of osmium, and also complexes of N macrocycles, are generally similar to those of Ru, but [Os(bipy)3]2+ and [Os(terpy)2]2+ are more labile and reactive than their Ru analogues. As for Ru, some Os11 complexes are luminescent and potentially of use in photochemical molecular devices and for detection of DNA.66... 

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